This invention relates to an overspeed safety device for a pneumatic motor. In particular the invention relates to a safety device comprising a rotating annular valve disk provided with means responsive to centrifugal action to shift the valve disc from an open position to a closed position at a predetermined speed level.
In certain pneumatic motor installations, as for example in grinding machines, it is of greatest importance that not only the machine but in particular the working tool connected thereto is effectively prevented from overspeed at idle running. If the motor speed in grinding machines is allowed to increase above a certain level there is great risk that the grinding tool, when exposed to severe centrifugal forces, will break up into pieces and, thereby, put personnel and equipment in danger. In order to avoid the risk of damage due to overspeed, machines of this type are equipped with speed governors.
Speed governors for this purpose, however, are mechanical and under certain conditions, when exposed to hard wear, rust and dirt, they easily get out of order.
In order to obtain a safe protection for personnel and equipment there have been suggested overspeed safety devices which act independently of the speed governors to shut off the pressure air supply to the motor at a speed level above the normal speed governor activation level.
A safety device of this type is previously shown and described in German Patent Publication DOS 2303942. This prior art safety device comprises a conical steel disk attached to the rotating spindle of a pneumatic motor and provided with a number of centrifugal weights rigidly attached along the periphery of same. As the spindle and the valve disk reaches a predetermined speed level the centrifugal action upon the centrifugal weights forces the steel disk to snap over and assume the shape of a cone facing the opposite direction. In its latter position the steel disk covers the air inlet of the pneumatic motor and interrupts further pressure air supply to the motor.
This known device is characterized by its frictionless action and that the speed level at which the device has to be closed is determined just by the shape or pretension of the steel disk and the mass of the centrifugal weights mounted on the disk. It is an advantage for a mechanism like this to operate frictionless, but a disadvantage inherent in this device is the difficulty in accurately predetermining the shut-off speed level. Another drawback of this known device is that, since no positively acting holding means is associated with the valve disk, the latter might be shifted from open to closed position or vice versa by external activation, like for instance a blow on the outside of the housing in which the device is enclosed.
The object of the present invention is to overcome the above mentioned problems and create a valve which is exposed to a minimum of friction and which is activated at a very well defined speed level.